Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri May 12 2023 08:26:54 FOUS30 KWBC 120826 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 426 AM EDT Fri May 12 2023 Day 1 Valid 12Z Fri May 12 2023 - 12Z Sat May 13 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTH-CENTRAL TEXAS... ....Texas... A significant flash flood event could evolve Friday night across portions of south central TX. Forcing, moisture and instability all look to be in place to support an excessive rainfall event. Forcing is being driven by a mid level shortwave moving out of Mexico and broadly diffluent flow aloft in between the subtropical jet to the south and a jet streak to the north of the area. This mid/upper forcing persists through the period, and does not show much eastward progression with time. This type of persistent forcing evolution often favors training/backbuilding. Strong southeasterly low level flow off the Gulf of Mexico and a mid/upper level connection to the Pacific will support anomalous PWs and efficient rainfall. Instability should be on the order of 3000-4000 J/kg, with plenty of upstream instability to maintain convection. The main question is the last remaining ingredient for flash flooding...heavy rainfall rate duration. The aforementioned slow moving forcing supports prolonged duration, and deep layer southerly flow, combined with southeasterly low level flow, supports training/backbuilding. As convection organizes it should develop a cold pool and propagate eastward into the low level flow and downstream instability pool. Exactly how quickly this happens is the big question. Initial convection should develop along the Edwards Plateau as forcing and upslope southeasterly flow increases. Think we will see a period of backbuilding/training during this convective phase, before cold pool generation results in an east or southeast progression of activity. At this point leaning towards the slower HREF solutions, as some of the members have a tendency to become cold pool dominant too quickly. Thus would not be surprised if we end up on the higher end of the HREF output for QPF amounts. Thus still thinking this is an event where swaths of 6"+ rainfall are a possibility, with 2-3" an hour rates. Scattered to numerous instances of flash flooding, some of which could be higher end in nature seem possible within/near the Moderate risk area. Convection is also likely along the dryline over west central TX during the afternoon hours. This activity will initially be slow moving and should pose an isolated to scattered flash flood risk. Locally significant rainfall and flash flooding is possible with this activity as well, but coverage should be a bit lower than with the Hill Country activity, and thus a Slight risk should cover the threat. Although guidance has been trending up over this area, and so will need to continue to closely monitor. Overnight this dryline activity should survive eastward into central TX...but the exact magnitude and organization of this activity remains unclear. However a broad Slight risk seems warranted as a flash flood risk could continue into central TX overnight. ....Northern Plains... Isolated to scattered flash flooding is possible over a large portion of the Northern Plains today into tonight. A Slight risk stretches from NE into much of SD and into west central ND. A closed deep layer low moving slowly northward will provide plentiful forcing for convection. A stationary front over the western Dakotas will continue to be one focus for persistent convective activity. Instability is a bit more meager here, however strong low level easterly flow and persistent convergence will still support periodic rainfall totals over 1" in an hour. The 00z HREF supports additional widespread 1" rainfall totals here, with isolated to scattered 3"+ totals likely. The 2nd area of concern is closer to the deep layered low and associated warm front from NE into southeast SD and northwest IA. This activity will have more instability to work with, and thus 2"+ an hour rainfall is a possibility. This activity should be a bit more progressive, but do expect we will at least see some localized swaths of backbuilding/training. HREF probabilities also support isolated to scattered 3"+ rainfall totals here as well. ....MS Valley into the OH/TN valley and Southeast... A broad Marginal risk will cover a lot of this region today/tonight. Would have liked to try to narrow this area down, but found it hard to do, hence the very large Marginal. Above average moisture and instability will cover this entire region, so any convection will be capable of heavy rainfall rates, and where convection persists, could result in localized flash flooding anywhere within this risk area. The actual coverage of FFG exceedance will probably be pretty low over this area, but localized instances could occur just about anywhere within the risk based on 00z HREF and other QPF guidance. Given this HREF output and the favorable moisture/instability parameters...thought going with a large Marginal was the best course of action. Chenard Day 2 Valid 12Z Sat May 13 2023 - 12Z Sun May 14 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR MUCH OF SOUTH TEXAS... ....Texas... The event discussed in the day 1 discussion should be ongoing at 12z Sat across portions of south TX. The expectation is that the convective complex will be making slow eastward progress by this time. The forecast by this time is already becoming lower confidence, as convection on day 2 will be heavily dependent on the location of cold pool driven outflows and ongoing convective activity. Guidance has trended a bit quicker with convective progression, so we did trim back some on the western extent of the Moderate risk with this update. The overall synoptic environment will be conducive to excessive rainfall over central/southern TX, so really comes down to outflow and instability recovery, which will be tough to pin down at even this lead time. Do think the greatest flash flood risk will be over south TX associated with the likely continuing convective complex, but some renewed development behind this can not be ruled out either. Just like on day 1, extreme rainfall rates of up to 3" in an hour seem probable with this convection over south TX. Still thinking the potential is there for 6"+ rainfall amounts in spots across south TX downstream of the day 1 convection. Think this is a solid MDT risk level event, with locally significant flash flooding possible. Can not rule out a High risk level event, but the trend for a slightly quicker convective progression seems to more solidly put this more in the MDT level for now. A second area of flash flooding is possible across portions of north TX into OK associated with retrograding mid level shortwave and persistent upper level diffluence. Just like over south TX, we have some questions regarding the degree of instability and boundary locations here as well. Seems like a solid Slight risk level event with some backbuilding/training potential, and an embedded more organized threat could exist. Just too early to pin that down at the moment, but will continue to monitor trends. ....Northern Plains into Upper MS Valley... The closed mid level low will finally begin to open up into an open trough on Saturday, however enough forcing will persist to drive a convective threat near the stationary/warm front. It seems likely that we will get upwards of 2000 J/kg of instability into this boundary, which will be enough to support robust convective development near the front where convergence is maximized. Overall the ingredients appear to be in place to drive a heavy rainfall threat, and now model QPFs continue to suggest a narrow heavy rainfall event as well. Even through it only goes through the first half of day 2, the 00z HREF is already showing 3"+ neighborhood probabilities over 40%, and 00z ECMWF and GEM Reg QPFs are locally over 3" as well. This QPF support, combined with the favorable ingredients, increases confidence in a narrow heavy rainfall event. On top of that, it currently appears like there will be decent overlap with the portion of southern MN and northern IA that just recently saw heavy rainfall and some flooding impacts. Given all this, a Slight risk upgrade was warranted...with isolated to scattered flash flooding seeming more likely Sat into Sat night. ....OH/TN valley into the Mid Atlantic... Above average moisture and instability will cover portions of this region on Saturday. Guidance is also indicating a weak shortwave which could help focus convective development. The details remain unclear, and convection may end up fairly quick moving, however felt like the overlapping moisture/instability and convective signal in the 00z CAMs supports the potential for localized flash flooding. Chenard Day 3 Valid 12Z Sun May 14 2023 - 12Z Mon May 15 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN TEXAS... Forecast becomes pretty tricky by this time across the Southern Plains into the OH/TN valley. The overall synoptic setup over TX/OK will remain conducive for heavy rainfall, however exact magnitudes/locations are likely at least partially dependent on boundaries laid down by convection on day 2, and where instability is sufficiently worked over or not. Thus really hard to pin own specifics at this point. For the ERO really relied on ensemble data for the most part, with the GEFS, ECENS and CMCE all showing highest rainfall probabilities over south TX, where our Slight risk is outlined. This area seems to have the greatest potential for excessive rainfall, but also has bust potential depending on the above mentioned factors. A second area of potential heavy rainfall exists from northeast NM into north TX and into OK. This should be closer to the lingering shortwave feature and lower level synoptic boundaries. However the question here is how much instability remains this far to the north and west. Certainly some potential for Slight risk upgrades here, but for now thought the best solution was to place the Slight risk where ensemble QPF and ingredients have the higher end potential, and cover the remaining areas with a broad Marginal risk. The broad eastward Marginal expansion into the MS valley into OH/TN valley is primarily driven off of sufficient CAPE/PW overlap. Forcing will generally be weak and partially related to convective driven vort centers, which will be hard to narrow down at this lead time. So generally followed the overlap of favorable ingredients, and most of this area is within a QPF max in at least one model. A stronger shortwave is forecast to move through the OH valley by Sunday night, and it is possible that this could help drive a more organized convective threat. However at the moment the core of this forcing seems to be positioned north and east of the instability pool, so will just have to wait and see how much overlap we end up between the greater forcing and instability. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6T0IAxCakhSLmyjhhRH7KSOUyKbBcM3iV8KHBKWwBGR8= xeBTuVfWRRVhvpaVH-_3quB6Igl0V5SG7t8pzQle4y-x4T4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6T0IAxCakhSLmyjhhRH7KSOUyKbBcM3iV8KHBKWwBGR8= xeBTuVfWRRVhvpaVH-_3quB6Igl0V5SG7t8pzQlemqkYI1U$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6T0IAxCakhSLmyjhhRH7KSOUyKbBcM3iV8KHBKWwBGR8= xeBTuVfWRRVhvpaVH-_3quB6Igl0V5SG7t8pzQle64CZHoQ$=20 $$ =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .